Iron and magnesium beside a quasar: a possible trace of a primordial supernova

A stellar explosion in the early universe may have left a chemical trace around a distant quasar. The clue is not an image of the star or its explosion, but a ratio between two elements measured in the light from the gas surrounding the quasar ULAS J1342+0928. In an informational post dated September 28, 2022, NOIRLab explained why this composition led researchers to propose a possible pair-instability supernova, a type of explosion associated with very massive stars of the first cosmic generation.

The interpretation connects a spectroscopic observation with models of how stars die and produce elements. But NOIRLab’s account itself presents it as a possibility that needs further observations to be verified.

A quasar from the early universe

A quasar is a very distant object whose surroundings can be studied through the light we receive. The object analyzed, ULAS J1342+0928, is among the oldest and most distant known quasars. According to NOIRLab, the light we observe was emitted about 13.1 billion years ago, less than 690 million years after the Big Bang.

In this context, searching for signals from the first stars is one way to investigate how chemical elements appeared. Astronomers call stars from this first generation Population III stars. The source explains that the name distinguishes stellar populations ordered backward in time: Population III refers to the oldest stars.

The analysis was based on a spectrum obtained with the Gemini North telescope in Hawaii, using its GNIRS infrared spectrograph. A spectrum separates light into its component wavelengths. Some features at these wavelengths make it possible to identify which elements are present in the observed material. Estimating how much of each element is present is more complex: the brightness of a signal also depends on other factors, not just the abundance of the element producing it.

The chemical clue: iron and magnesium

To interpret the signals, the team used a method developed by Yuzuru Yoshii and Hiroaki Sameshima in 2017 to convert the brightness of spectral features into estimates of abundances. Applying it to the quasar’s spectrum, they found an iron-to-magnesium ratio approximately ten times higher than the solar ratio. Put another way, the relative amount of magnesium compared with iron was unusually low.

The comparison matters because, according to the explanation reported by NOIRLab, theoretical models of a pair-instability supernova predict a low magnesium-to-iron ratio. The composition of the gas around the quasar was therefore suggestive: it could contain material enriched by an explosion of this kind.

The proposal was not based solely on the chemical ratio. The quasar formed very early in the history of the universe, when there would have been less time for material from different explosions to mix. The combination of this great age and the chemical signal prompted the suggestion that the surrounding gas could contain remnants of an explosion of a Population III star.

What a pair-instability supernova means

According to NOIRLab, this type of supernova is thought to be a possible natural end for stars with masses between 150 and 300 times the mass of the Sun. The mechanism described begins in the stellar core: photons turn into pairs of electrons and positrons, particles of matter and antimatter. This transformation reduces the energy that supports the star against its own gravity. Gravity can then prevail, trigger collapse, and set off the explosion.

The team compared the quasar’s signal with models of these explosions. Yoshii noted that a supernova from a star of about 300 solar masses produced a magnesium-to-iron ratio consistent with the low value derived for the quasar. This agreement supported the interpretation, but did not by itself identify an explosion that had been directly observed: the evidence described is the composition of the material surrounding the quasar.

The hypothesis also connects to a broader question: how the elements now found in stars and other cosmic objects formed. If an explosion of a primordial star enriched nearby gas, its composition could preserve information about a very early stage of stellar evolution. In this case, that possibility is inferred from the abundances and the quasar’s ancient context.

An interpretation that still needs testing

Yoshii and his collaborators considered other explanations and concluded that a pair-instability supernova from a Population III star was the most likely interpretation. However, NOIRLab’s text does not present this reading as definitive confirmation: the authors said that many more similar observations are needed to verify the result with certainty.

Caution is also important because of how the clue is obtained. Abundances are estimated from the brightness of spectral features, and that brightness depends on more than the amount of each element. The reasoning therefore runs from a signal in the light to a chemical ratio and then to a comparison with models of stellar explosions. Each step supports an interpretation, not a direct observation of the star that is thought to have exploded.

The result highlighted by NOIRLab is therefore a possible chemical trace of a primordial supernova in the surroundings of ULAS J1342+0928. Its significance is that it offers a way to search for the remnants of first-generation stars in very ancient objects. Confirmation, according to the researchers cited by the source, depends on obtaining more comparable observations.

Information source: NOIRLab, “A glimpse of the first stars that lit up the Universe?”, September 28, 2022.